The present invention relates to level measurement in industrial processes, wherein the invention is used for measurement of product level in a storage tank of the type used in industrial applications using a microwave level gauge. More specifically, the present invention relates to a device and a method for efficient use of power provided to the gauge from a two-wire process control loop.
Instrumentation for the measurement of product level (either liquids or solids) in storage vessels is evolving from contact measurement techniques, such as tape and float, to non-conduct techniques. One technology is based on the use of microwaves, which involves transmitting microwaves towards the product surface and receiving reflected microwaves from the surface. The reflected microwaves are analyzed to determine the distance that they have travelled. Knowledge of the distance travelled allows determination of the product level.
Often used in the process control industry are the 4-20 mA process control loops. In these loops a 4 mA signal represents a zero reading and a 20 mA signal represents a full-scale reading. Further, if a transmitter in the field has sufficiently low power requirements, it is possible to power the transmitter using current from the two-wire loop. However, microwave transmitters for level gauging in the process control industry have often required a separate power source. These microwave transmitters were large and their operation required more power than could be delivered using 4-20 mA standard. Thus typical prior art microwave transmitters for level gauging required additional wiring to provide power to the unit.
The document U.S. Pat. No. 5,672,975 discloses an arrangement for providing power to a radar level gauge and for transmitting level information provided by the radar level gauge by means of a two-wire process control loop. The term radar level gauge is here used for a unit including an antenna unit, a microwave transmitter, a receiver, transmitter and receiver circuits and circuits for calculating a measured level.
A two-wire radar level gauge is distinguished by that it is being supplied by power and at the same time communicating analogue and digital information through the same wires. A prior art two-wire radar level gauge can be coupled as is shown in FIG. 1. A voltage source 1 is supplying the radar level gauge 2 with power through the two-wire loop 3. A barrier 4 for protection against current transients and for containing an EMC-filter may be included in an interface between the gauge and the loop. The gauge is conveying an actual measured value to a control unit 5 by setting a current proportional to the level value measured. This current can be set in the interval 4-20 mA. An antenna unit 6 is included in the gauge.
To make it possible to span the loop current over the whole 4-20 mA interval, the internal power consumption of the radar level gauge must be lower than or equal to 4 mA. This lowest limit is valid for a measured value that is represented by the lowest value to be conveyed by the loop. In reality not even as much as 4 mA current from the loop is available for powering the gauge. Current available for the supply of the gauge is approximately 3,5 mA. The reason for this is that the gauge is set to send a low level alarm when the reading is below the lower end of the current range 4 to 20 mA used for transmission of data via the loop.
A two-wire loop is characterized by that only two wires are needed for connecting an instrument to a controller. The length of the wires may be up to several hundreds of meters. The wires are also used for the communication between the instrument and the controller. Said communication may be analogue in the 4-20 mA interval as stated. Digital bi-directional communication according to e.g. the known HART protocol is also possible to use in said interval.
Equipment which is to be located in explosive environments is subjected to authority demands. It is common that equipment is then designed and certified as xe2x80x9cexplosive safexe2x80x9d or xe2x80x9cintrinsic safexe2x80x9d.
Safety against explosion is guaranteed, in general, by use of a casing, which complies with certain requirements. Such equipment may be powered via a barrier to limit the energy that is fed out to the wires and to the gauge. Intrinsic safe means that the construction in itself is designed in such a way, that electric energy is not available in a sufficient amount to generate a spark, which can set fire to an explosive gas surrounding the construction. From practical reasons, this means that there is a barrier at the entrance to those parts being classified as intrinsic safe. Either a barrier 4 or a barrier 7 exemplifies this.
For a radar level gauge, parts of the equipment that must be located inside, for example, an oil container has to be intrinsic safe. As a result there is required a barrier that limits, with high security, energies possibly available at wave guides and antenna parts.
The input voltage to the gauge at the loop end changes depending on the barrier used, supply cable characteristics, loop current, losses in the gauge and supply voltage. A loop power supply is located remote from the gauge, often at the same location as the receiver of data from the gauge. A typical remote supply voltage is 24 V and the total resistance in the loop is often 500 ohms or more. Consequently, the input voltage at the gauge terminals can vary within a 10 V interval as the voltage drop across the loop can reach 10 V at 20 mA in the loop.
Document SE 0203456-9, not yet published at the filing date of this application, discloses the use of a DC/DC-converter for supplying the gauge with power from the loop. The content of said document is hereby incorporated into this description by reference. FIGS. 3 and 4 illustrate a converter according to the disclosure of said document. The converter of said disclosure further has the purpose to isolate the gauge from the loop. The total power consumed in the gauge is Uloop*Iloop. See FIG. 4 (FIGS. 3 and 4 are described more in detail in the embodiments below). The total current Iloop is divided into the current IDC/DC, which is used for powering the measurement circuits of the gauge, and a shunt current Ish. Said total current Iloop is the current corresponding to the value measured by the gauge. As seen from the loop, the input terminals of the converter of FIGS. 3 and 4 are the input terminals to the gauge. The power consumed is used to energize the gauge. The amount of power required to supply the gauge is usually a portion of the total consumed power. A converter of the kind used to transfer power from the loop to the gauge as shown has a built-in feature that the input power to the converter is fairly constant over a wide input voltage range. Excessive power is dissipated in a device, a current generator, controlled by the gauge to stabilize the loop current to an amplitude that corresponds to the present reading of product level. Limitations arise when the power required by the gauge exceeds the available power from the loop. In reality, this available power sets a limit, a threshold, for minimum operational input voltage for the gauge. Said threshold is here called xe2x80x9clift-off voltagexe2x80x9d. The term lift-off voltage is used to indicate the voltage level that must be available for the gauge to perform as defined at a certain current. If the available voltage for the gauge is below the required lift-off voltage at said certain current the power is not high enough to energize the gauge. The value of the lift-off voltage is generally lower when high loop current is prevailing as the loop can supply required power without limitation.
A way to solve the problem with insufficient power to the gauge is described in document U.S. Pat. No. 6,014,100. In said document the measurement of the level of the product is made in an active cycle between energy storing cycles. The transmission/receive unit is completely switched off during a temporary power store cycle and is inoperative until the start of the next active cycle. A drawback with this solution is that the transmission/receive unit is unavailable during energy storing cycles.
Another way to solve the same problem in a gauge is shown in document U.S. Pat. No. 5,416,723. In said document an invention is disclosed, where a power regulating circuit is associated with a circuit configured to sense a deficit in its capability to supply the circuits of the gauge with power and to delay the execution of a program stored in the gauge for control of the measuring of the level by the gauge. The program is delayed sufficiently in response to the sensing of the deficit to reduce the power required by the gauge to overcome the deficit. A drawback of the disclosure is further that a loop current setting means is connected in series with the transducer, whereby the full loop current is passing said setting means and thus imparting a voltage drop across said setting means in the loop.
Patent application U.S. Pat. No. 2002/0005713 A1 discloses a device, a gauge powered from a loop, which shows a way to approximate the power consumption of a gauge to predetermined power consumption without the predetermined power consumption being exceeded. The measuring operations are then regulated in a way such that a power surplus is minimized.
The object of the present invention is to provide a circuit for a radar level gauge powered via a two-wire control loop for sensing the surplus power transferred in the loop and for use of said surplus power in increasing performances of the gauge.
According to one aspect of the invention, there is presented a two-wire powered radar level gauge comprising the features:
the radar level gauge uses microwaves and is coupable to a two-wire process control loop for measuring a level of a surface of a product in a tank and the radar level gauge further comprising,
a microwave antenna unit directed into the tank,
a microwave source for sending a microwave signal through the antenna unit into the tank,
a microwave receiver for receiving a reflected microwave signal from the surface of the product in the tank,
measurement circuitry coupled to the source and receiver for initiating transmission of the microwave signal and for determining product level based upon the received signal,
output circuitry coupled to the two-wire process control loop for setting in the loop a desired value of a loop current (Iloop) corresponding to the product level,
a power supply circuitry coupled to the two-wire process control loop for receiving power from the loop and being a source of power for the microwave source, the microwave receiver, the measurement circuitry and the output circuitry and including a converter for transferring power from the loop to said power supply circuitry by means of feeding a first current (IDC/DC) from the loop to the power supply circuitry,
a current generator included in said output circuitry for generating in parallel to said first current a second current (Ish) in the loop for maintaining said loop current at a current level corresponding to said product level and
a sensing circuit for determining the value of said second current and having an output indicative of the value of said second current.
According to a second aspect of the invention, there is presented a method for measuring a level of a surface of a product in a tank by means of a radar level gauge using microwaves, wherein said gauge is coupled to a two-wire process control loop and the method further comprising the steps:
directing an antenna unit into the tank,
sending by means of a microwave source a microwave signal through the antenna unit into the tank,
receiving by means of a receiver a microwave signal reflected from a surface of the product in the tank,
initiating transmission of the microwave signal and determining product level based upon the received signal in a measurement circuit coupled to the microwave source and the microwave receiver,
setting in an output circuitry situated in the radar level gauge and coupled to the two-wire process control loop a loop current value (Iloop) corresponding to the product level,
transmitting information related to product level over the loop via said output circuitry,
supplying power from the loop to a power supply circuitry being a source of power for the microwave source, the microwave receiver, the measurement circuitry and the output circuitry,
transferring power from the loop to said power supply circuitry by means of feeding a first current (IDC/DC) from the loop to the power supply circuitry,
generating a second current (Ish) in the loop for maintaining said loop current at the value corresponding to said measured value of said product level,
determining in a sensing circuit a value of the current generated in the loop and sending an alarm via the loop when the value of said generated current indicates that the power available via the loop is insufficient to energize the gauge and outputting from said sensing circuit an output signal indicating the value of said generated current.
The invention uses the sensing circuit, which senses current generated in said current generator for allowing excessive power available to be used for powering equipment in the radar level gauge. In one embodiment the sensing circuit is used for switching the gauge between two modes, one base mode for providing basic measuring functions with power and one extended mode, wherein further functions are allowed to operate.
The converter included in the level gauging system is transforming the voltage from the loop to an over-voltage protected and current limited low feed voltage to the electric circuits of the gauge. The converter can at the same time be supervised to load the loop in said extended mode with currents between 4 and 20 mA. Process signals (such as HART signals) from and to the gauge can be handled by the circuit (not shown) and transmitted in both directions.
With the sensing circuit it is possible to determine when lift-off voltage is reached, whereby it will be possible to utilize power available from the loop more efficiently. The gauge can be made to adapt its function or performance depending on available power supplied. The gauge can also be operational from lower input voltage. Specific areas where the detected available power can be used are the powering of circuits for:
Controlled start-up and shut down of the level gauging system,
Controlling slow-down processes in the gauge to lower the power consumption,
Activating stand-by modes,
Controlling connection and disconnection of certain functions or to switch to alternative modes of operation to affect the power consumption and by this offer an extended input voltage interval for the gauge,
Starting optional functions,
Charging electrical storage means, such as a capacitor or a battery, in the gauge with sensed excessive power.
Discrete measures that can be fulfilled with the circuit according to the invention, when the sensing circuit activates an alarm indicating that the lift-off voltage is reached:
Clock speeds in digital circuits can be changed
The measurement updating intervals can be changed
Parts or functions in the gauge, which are not necessary for the basic functions of the gauge, can be disconnected,
Functions used for service or for maintenance of the gauge can be disconnected.
The circuit according to the invention aspects can be used for a better control of the gauge when the supply voltage is switched off or if the voltage for a short while would fall outside normal levels. Situations where circuits are locked due to uncontrolled high current level during e. g. start can be eliminated. Setting the sensing circuit to connect a low power stand by state of rest when the power supply cannot fulfil the demands for normal performances can provide this function.
Further advantages are that the level gauging system can be programmed to use available energy for optimum operational performance at any time and that the sensing circuit can be used for supervision of loop supply. The lift-off voltage can be detected at any loop current.
The proposed circuit increases the efficiency of the gauge compared to a conventional solution as functions of the gauge can be controlled in such a way that the voltage interval in which the gauge will work is expanded.